Traditionally, powdered products such as joint compounds, cement, cocoa, flour and the like, have been packaged in paper bags for use with high-speed filling and forming machines. However, there are many drawbacks associated with the use of paper bags. Paper bags are not water-resistant. If exposed to water or to humid conditions, the paper absorbs the water, often transferring it to the contents of the bag. If the contents include cement or gypsum, for example, the introduction of water can allow the material to set, rendering it inactive for later use. Paper bags also lack strength. They are punctured or torn relatively easily, allowing the contents to spill out and be lost.
Attempts have been made to utilize plastic bags for powdered products due to their higher strength and water resistance. When non-porous plastic films are used to keep water out, residual air that is inside the bag at the time it is sealed is trapped inside. Backpressure that is created upon filling causes the bags to acquire balloon-like appearance. In many cases, bags are underfilled due to the product being blown out of the bag during automatic filling. The ballooned bags take up additional space for storage and shipping, can be unstable when stacked, compromise the heat seals and reduce the overall efficiency and cleanliness of the production line. The use of suction to remove the excess air often draws a portion of the product with the removed air.
Processes and equipment have been developed that remove much of the air from a plastic bag prior to sealing, but the current technology is limited to about four bags per minute. This rate is considerably less than the ten bags per minute that can be achieved with paper bags in a conventional Form/Fill/Seal process.
In order to overcome this problem, polyvinylchloride bags have been perforated with needles to provide openings through which the residual air can escape. Even relatively thin needles result in perforations of about 1,000 μm, a size that is relatively large compared to the 10 μm to about 50 μm particle size of fine powders. During packaging and handling, the powders can escape through the perforations, creating a mess and loss of product. Moreover, the needle perforations varied greatly in diameter and had ragged edges, sometimes causing the holes to plug and hinder the escape of residual air.
A plastic foil bag with laser-formed venting perforations is disclosed in U.S. Pat. No. 4,743,123. The foil wall is perforated by laser radiation. The perforations range in size from about 50 μm to about 150 μm. Spacing of the perforations must be chosen to preserve the strength of the foil. Moisture, and at times product, enters and exits the bag through the perforations. Even when two layers of bags are used and the perforations are staggered, air and contaminants have a longer, more tortuous path to follow, but they still can enter the bag.
In U.S. Pat. No. 6,126,975, a bag is disclosed having a flap over the microperforations. In the manner of a petal or check valve, when entrapped air leaves the bag, the flap is blown out of the path, but then the flap settles down over the pores when air is no longer coming from the bag. However, this flap is easily pushed aside by friction against adjoining bags, or can even be torn off. As with the two layer bag, air, moisture and product can still enter and exit the bag.
There is, therefore, a need in the art for a strong bag for powdered materials that can be formed and filled at rates comparable to those of paper bags. Another need exists for a bag that allows residual air in the bag to be expelled at a rapid rate. Yet another need exists for a water resistant bag for fine powders that are degraded by premature exposure to moisture.